1. Technical Field
The present invention relates to a fixing device, a fixing device control method, and an image forming apparatus, and more particularly, to a fixing device for fixing an image in place on a recording medium, a control method for use in such a fixing device, and an image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of those features.
2. Background Art
In image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by transferring ink or toner onto a recording sheet such as a sheet of paper. The transferred, unfixed toner image may be subsequently subjected to a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium with heat and pressure.
Various types of fixing devices are employed in electrophotographic image formation. In general, a fixing device includes a combination of rotary fixing members, such as rollers and belts, one being a fuser member subjected to heating, and the other being a pressure member pressed against the heated one to form a fixing nip therebetween, through which a recording medium is conveyed to melt and fuse a toner image in place under heat and pressure as the rotary fixing members rotate together. Typical methods include a roller-based method using a cylindrical fuser roller, and a belt-based method using an endless fuser belt entrained around one or more cylindrical rollers. The fuser member is equipped with a suitable heat source, such as a resistant heating element, a halogen heater, an induction heater, or a magnetic heater, from which heat is supplied to the fixing nip.
To date, several fixing assemblies have been available that employ a positioning mechanism to position the pressure member into and out of contact with the fuser member. The positioning mechanism allows for creating a gap or spacing between the fuser and pressure members during idle or standby where there is no recording medium passing through the fixing device. Once the fixing process is back to normal operation, the spacing is removed by bringing the pressure roller into contact with the fuser member to reestablish the fixing nip.
Provision of spacing or gap between the fuser and pressure members effectively prevents imaging defects, in particular, toner blistering, caused due to overheating of the pressure member. Toner blistering occurs where moisture contained in a recording medium evaporates into vapor bubbles during thermal processing, which eventually form swelling or blisters on a toner image being fixed on the recording medium. The problem is often encountered during duplex printing where a recording medium, having its first surface initially printed and second surface secondly printed, passes through the fixing nip to fix the secondly printed image, while brought into contact with the pressure member that imparts a substantial amount of heat to cause moisture evaporation, resulting in toner blisters on the initially printed image. Toner blistering makes the resulting image appear rough and uneven, which detracts much from imaging quality of the fixing process.
In order to effectively prevent toner blistering with the positioning mechanism, the pressure member is required to be sufficiently spaced apart from the fuser member, so as to isolate the pressure member from thermal influence, i.e., radiation and other forms of heat transfer, from the fuser member, which would otherwise interfere with efficient cooling of the pressure member. To this end, operation of the positioning mechanism may be controlled according to readings of a thermometer measuring temperature on a surface of the pressure member, such that the pressure member do not accumulate excessive heat from the fuser member, as the latter is typically intensively heated during operation.
One problem encountered when using the positioning mechanism is a prolonged first print time required to complete an initial print job upon activation of the image forming apparatus. That is, creating a gap or spacing between the fuser and pressure members necessitates restoring the pressure member into its operational position from non-operational position as the fixing device returns from an idle or standby state. Restoration of the operational position, however, takes a substantial period of delay time, such as on the order of several to more than ten seconds or even longer, depending on driving equipment employed, since the pressure member is required to move gradually and gently toward the fuser member, so as to prevent striking the pressure member against the fuser member, which would otherwise cause undesired vibrations to propagate through neighboring structures, eventually causing adverse effects on the resulting print. This delay, if taking place after a user submits a print job upon activation of the image forming apparatus, can translate into a first print time longer than would otherwise be expected.